The aim of the proposed study is to exploit the known specificity of the cystine requirement characteristic of certain human tumor cell lines. A variety of microorganisms will be screened as producers of Beta-cystathionase and growth conditions for maximal yields and properties of this enzyme will be defined. Two good producing strains have been already identified. Those Beta-cystathionases having the most favorable stability and kinetic properties with respect to degrading L-cystine and L-L-cystathionine will be prepared in quantity. The enzyme(s) will be administered to groups of mice inoculated with L-1210 lymphoma and fibrosarcoma cells, and the incidence and degree of tumorgenesis will be observed and compared with similar groups receiving no enzyme and uninoculated groups. The degree of cystine depletion attained will be correlated with the diminished tumor growth. Approaches to lowering tissue levels of glutathione will be considered for use in conjunction with the enzyme treatment. If results are encouraging, the enzyme will be modified chemically in attempt to enhance its efficacy.